In an election dominated by concerns about the economy, taxes have become the endlessly-invoked Republican symbol of limited government. Mitt Romney was lambasted for allowing a surrogate to say that the Affordable Care Act represented a penalty, not a tax, a statement that ceded a crucial attack line and earned a rebuke from the Wall Street Journal editorial page.

The latest Crossroads ad buy seeks to link several vulnerable Democrats in tough Senate races to tax increases. Spots set to run Virginia, Missouri, Montana, North Dakota and Nevada portray Democrats as proponents of higher taxes and needlessly expanded government.

An ad attacking Rep. Shelley Berkley is the exception, instead targeting the Nevada Democrat for a House probe into allegations that she used her position to help her husband's business prospects.

But the prospect of higher taxes is the recurring bogeyman in the other ads, particularly for incumbents who voted in favor of the health care overhaul. Here's a sampling:

Claire McCaskill, Missouri: The imperiled first-term incumbent "sides with government," the spot says, having "voted repeatedly for higher taxes on on nearly half a million job-creating Missouri businesses" and supported the Affordable Care Act.

Jon Tester, Montana: Tester "votes to raise taxes on Montana families and small businesses" and "was the deciding vote to pass the heath care law, which we now know is a massive middle class tax increase," according to an ad entitled "Sense."

Heidi Heitkamp, North Dakota: "What's Heidi hiding on taxes?" the spot "Hiding Taxes" wonders, adding that the former attorney general had supported new taxes on car insurance and energy.

Tim Kaine, Virginia: The former governor is battling Republican George Allen, who is seeking to reclaim his former Senate seat, and the Crossroads Ad hammers Kaine for having supported the federal stimulus package and backing "massive tax hikes every year" while in the governor's mansion. "Kaine's failed solutions: tax hikes. Wasteful spending," the ad says.